1. Field of the Invention
The present invention is directed to a planetary steering system for a tracked vehicle, wherein the speed of the left and right driving wheels can be independently controlled.
2. Description of the Prior Art
There are three basic methods for steering skid-steered vehicles. These methods include (1) clutch/brake systems, (2) hydrostatic systems, and (3) planetary steering systems. In clutch/brake systems, the inner drive wheel in a turn is slowed or stopped by brakes as the outer drive wheel continues to rotate. In hydrostatic systems, hydraulic motors drive the drive wheels independently of one another. In a hydrostatic system the inner drive wheel in a turn can be slowed, stopped or reversed, and the outer drive wheel speeded up. In hydrostatic systems these operations maybe combined as desired. In a typical planetary steering system, a single hydraulic motor subtracts speed from the inner drive wheel and adds speed to the outer drive wheel when making a turn. The single hydraulic motor engages left and right planetary gearing assemblies associated with each respective drive wheel.
Dual hydraulic motor planetary steering systems have been proposed, see U.S. Pat. No. 4,917,200. Of particular interest is the embodiment illustrated in FIG. 3 of this patent. In this embodiment the prime mover drives an axle that forms the ring gear of port and starboard planetary assemblies. The sun gear of each planetary assembly is driven by the respective port and starboard hydraulic motors. The output shafts of the planetary assemblies are connected to the port and starboard drive wheels. The hydraulic motors are coupled to the sun gears by locking worm gear assemblies.
U.S. Pat. No. 5,139,465 discloses a two-path hydromechanical transmission system. At low tramming speeds the transmission system utilizes a two-path hydrostatic transmission. At high tramming speeds the system uses a two-path hydromechanical transmission. In high tramming speed operations, the speed of the hydrostatic motors are differentiated for steering.